Magnetic contribution to outer-disk rotation in the spiral galaxy NGC 5055

Abstract We test whether fields of galactic strength enhance NGC 5055’s outer gas rotation. Finite analytic disk decompositions yield consistent rotation–density pairs and a blended gravitational reference approximately retaining inner rotation while its density approaches the measured outer H i+He profile. This reference is modelled, not an independently measured stellar curve. Its difference from fitted gas rotation gives the required purely toroidal field for circular, axisymmetric midplane motion. The prescribed field strength at the reconstruction’s inner radial boundary is Bs. With hydrogen inertia, no pressure and Bs = 0, constant exponential heights of 1 kpc and 250 pc give approximate peaks of 5.47 and 10.95 μG near 34.9 kpc; 0.2 kpc gives approximately 12.24 μG. Extrapolated flaring exponential and Gaussian profiles peak at approximately 5.25 and 5.96 μG, respectively, near 33.8 kpc. For $B_s=8{\!\, }\mu \rm {G}$, their fields decline overall to approximately 5.73 and 6.30 μG near 39.7 kpc. These estimates are conditional on the gravitational reference, vertical structure and prescribed boundary field. Comparison with total radio fields supports the field scale without establishing a magnetic origin for the rotation excess.